The present invention relates to electrical switch circuits, and in particular to methods and circuits for decreasing the turn-on time in switches having silicon controlled rectifiers ("SCRs") through the innovative use of MOS controlled thyristors ("MCTs").
Electrical circuits containing SCR switches are well known. Often in such circuits the switch turn-on time is limited by the capability of the SCR. An SCR typically has a relatively small portion of its active area that is initially turned on by a gate current, and thus has an initial forward voltage drop that is relatively large. The period during which the active area of the device becomes active is called the SCRs spreading time. During this time, the forward voltage drop is relatively large compared to the voltage drop when the device is fully on. For example, an 8,000 volt SCR may have an initial forward voltage drop as large as 1,000 volts at the beginning of a 15 to 20 microsecond spreading time. By the end of the spreading time the voltage drop diminishes to a sustained forward voltage drop of 2 to 3 volts. The SCRs large initial forward voltage drop during the long spreading time limits the utility of SCR switches in applications where a source current changes rapidly with time, that is, has a relatively high di/dt, such as found in high voltage, 60 hertz power generators and power transmission equipment. In higher frequency circuits, important because of their smaller capacitance and inductance, di/dt is proportionally higher and more limiting.
The relatively high initial forward voltage drop also causes a larger and potentially damaging loss of energy in the device during turn on. In high frequency applications, where the device is being repeatedly turned on and off, the total power lost during the turn-on transients may be significant. Relatively, SCRs are limited in their ability to take current quickly but are adept in handling large currents over long periods of time. These facts limit the usefulness of SCRs for switching for short pulses and pulses with high di/dt.
To overcome these problems, it is known to place plural SCRs in parallel. For example, if the required di/dt capability of a switch is 2,000 amps/microsecond, and each SCR has a di/dt capability of 200 amps/microsecond and a surge capability of 2,000 amps, ten of the SCRs may be placed in parallel in a switch circuit to achieve the desired result. The switch circuit, because of the paralleled SCRs, has a total di/dt capability of 2,000 amps/microsecond. Note, however, that any one of the SCRs would be sufficient to carry the entire surge capability. After one of the SCRs is fully turned on and is able to carry the full current, the remaining nine SCRs are no longer needed. They may be switched out of the circuit at a cost of additional circuit complexity or the devices may be left to carry a current far below their individual rated capabilities. However, this solution wastes resources, takes up space that may be put to better use and increases the likelihood of failure of the device by introducing more elements.
In applications using SCRs to shape fast rising pulses, it is known to use relatively high power to obtain a pulse which can be shaped to a fast rising, lower voltage pulse. Typically, the slope of the di/dt curve for a given device decreases as the power across the device approaches the device's rated power. Thus, to obtain a relatively quick rise time, it is known to use a device with a higher power rating than necessary for the continuous load in order to operate within the sharp rising areas of the device's operating characteristics. For example, a person desiring a fast rising 100 volt pulse may use a 1,000 ampere pulse through a 1,000 ampere rated SCR in order to provide a pulse that can be shaped into a 100 ampere, ten times faster rising pulse. In such circumstances, it would be advantageous to have available a device which has a relatively sharp rising pulse throughout its operating characteristics so that the pulsing circuit does not need to use, otherwise unnecessary, higher power rated devices merely to obtain an acceptable rise time.
MCTs are power devices that are turned on quickly over their entire active area and have a very large di/dt capability. A description of such devices and explanation of their fabrication process may be found in U.S. Pat. No. 5,111,268 to V.A.K. Temple, which is hereby incorporated by reference. For example, a 600 volt, 75 amp MCT has a di/dt of up to 10,000 amps/microsecond and may be turned on in about 0.2 microseconds. However, MCTs are, at present, more expensive to manufacture than SCRs and have found limited application replacing SCRs. MCTs may have a somewhat larger sustained forward voltage drop than high voltage SCRs because the MCT manufacturing process presently produces a shorter carrier lifetime. MCTs may also have a somewhat smaller active area than SCRs due to limitations related to gate oxide defect density in the manufacturing process. It would be advantageous from a di/dt capability perspective and from a power loss perspective to replace SCRs in many applications with MCTs. However, the limitations of present MCTs have as yet limited their use as full replacement for SCRs.
It is, accordingly, an object of the present invention to provide a novel method and circuit using MCTs and SCRs to improve the turn-on time and di/dt capability of an electrical switch circuit while retaining the circuit's performance when the switch is fully turned-on.
It is a further objection of the present invention to provide a novel method of reducing the turn-on time of a switch by initially conducting switch current through a switching device with a relatively fast turn-on and subsequently through a switching device with a relatively small sustained forward voltage drop.
It is yet a further object of the present invention to provide a novel electrical switch circuit in which an SCR is connected electrically in parallel with a MCT to improve the turn-on time of the switch circuit.
It is still a further object of the present invention to provide a novel semiconductor switch having an SCR and MCT connected in parallel and integrated in a single semiconductor device.
These and many other objects and advantages will be readily apparent to ones skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of preferred embodiments.